Crude oil is conventionally processed by distillation followed by various cracking, solvent treatment and hydroconversion processes to produce a desired slate of fuels, lubricating oil products, chemicals, chemical feedstocks and the like. An example of a conventional refinery process includes distillation of crude oil in an atmospheric distillation to recover gas oil, naphtha, gaseous products, and an atmospheric residuum. Streams recovered from crude distillation at the boiling point of fuels have customarily been used directly as fuels. Generally, the atmospheric residuum is further fractionated in a vacuum distillation unit to produce a vacuum gas oil and a vacuum residuum. The vacuum gas oil is commonly cracked to provide more valuable light transportation fuel products in a fluid catalytic cracking unit or by hydrocracking. The vacuum residuum can be further treated for conversion to more valuable products. For example, vacuum residuum upgrading processes can include one or more of residuum hydrotreating, residuum fluid catalytic cracking, coking, and solvent deasphalting.
There are generally three common reactor types used in the refining industry: fixed-bed, ebullated-bed, and moving-bed. The decision to use a particular type of reactor is based on a number of criteria including the type of feedstock, desired conversion percentage, flexibility, run length and product quality, among others. In a refinery, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams containing varying amounts of contaminants such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in vacuum gas oil, deasphalted oil, and residues.
In a fixed-bed reactor, catalyst particles are stationary and do not move with respect to a fixed reference frame. Fixed-bed technologies have considerable problems in treating particularly heavy charges containing relatively high quantity of heteroatoms, metals and asphaltenes, as these contaminants cause the rapid deactivation of the catalyst and plugging of the reactor. In conventional fixed-bed reactors, the hydroprocessing catalysts are replaced regularly in order to maintain the desired level of catalyst activity and throughput. Multiple fixed-bed reactors connected in series can be used to achieve a relatively high conversion of heavy feedstocks boiling above a cut point in the range of about 300-400° C., but such designs require high capital investment and, for certain feedstocks, commercially impractical, for instance, catalysts replacement every 3-4 months.
The ebullated-bed reactor was developed to overcome plugging problems commonly associated with fixed-bed reactors during processing of relatively heavy feedstocks and as the conversion requirements increases, for instance, for vacuum residue. In general, an ebullated-bed reactor includes concurrently flowing streams of liquids or slurries of liquids, solids and gas, through a vertically-oriented cylindrical vessel containing catalyst. The catalyst is placed in motion in the liquid and has a gross volume dispersed through the liquid medium that is greater than the volume of the mass when stationary. In an ebullated-bed reactor, the catalyst is in an expanded bed, thereby countering plugging problems associated with fixed-bed reactors. The fluidized nature of the catalyst in an ebullated-bed reactor also allows for on-line catalyst replacement of a small portion of the bed. This results in a high net bed activity which does not vary with time. Early ebullated-bed processes and systems are described by Johanson in U.S. Pat. Nos. 2,987,465 and 3,197,288, both of which are incorporated by reference herein.
Moving-bed reactors combine certain advantages of fixed-bed operations and the relatively easy catalyst replacement of ebullated-bed technology. Operating conditions are generally more severe than those typically used in fixed-bed reactor, that is, the pressure can exceed 200 Kg/cm2, and the temperature can be in the range of from 400° C.-430° C. During catalyst replacement, catalyst movement is slow compared to the linear velocity of the feed. Catalyst addition and withdrawal are performed, for instance, via a sluice system at the top and bottom of the reactor. The advantage of the moving-bed reactor is that the top layer of the moving-bed consists of fresh catalyst, and contaminants deposited on the top of the bed move downward with the catalyst and are released during catalyst withdrawal at the bottom. The tolerance to metals and other contaminants is therefore much greater than in a fixed-bed reactor. With this capability, the moving-bed reactor has advantages for hydroprocessing of very heavy feeds, especially when several reactors are combined in series.
Companies that develop ebullated-bed technology include: Chevron Lummus Global, Axens, Headwaters, Institut Francais du Petrole (IFP) Energies Nouvelles, Hydrocarbon Research Institute (HRI), City Services, Texaco, Hydrocarbon Technologies Inc. (HTI). The commercial names for the ebullated-bed technologies include: H-Oil, T-Star, and LC-Fining.
One major technical challenge posed when hydrotreating heavy oil fractions or whole crude oil is the effect of small concentrations of contaminants, such as, for example, organic nickel and vanadium compounds and poly-nuclear aromatic compounds. These organometallic compounds and others have been proven to reduce the activity or useful life of hydrotreating catalysts. The presence of such metal contaminants and poly-nuclear aromatics result in reduced process performance, increased capital costs and/or increased operating costs of refinery processing units. The metals in the residual fraction of the crude oil deposits on the hydroprocessing catalyst and results in catalyst deactivation. The poly-nuclear aromatic compounds are coke precursors and at high temperatures form coke, which also causes catalyst deactivation.
Heavier fractions from the atmospheric and vacuum distillation units can contain asphaltenes. Asphaltenes are solid in nature and comprise polynuclear aromatics, smaller aromatics and resin molecules. The chemical structures of asphaltenes are complex and include polynuclear hydrocarbons having molecular weights up to 20,000 joined by alkyl chains. Asphaltenes also include nitrogen, sulfur, oxygen and metals such as nickel and vanadium. They are present in crude oils and heavy fractions in varying quantities. Asphaltenes exist in small quantities in light crude oils, or not at all in all condensates or lighter fractions. However, they are present in relatively large quantities in heavy crude oils and petroleum fractions. Asphaltenes have been defined as the component of a heavy crude oil fraction that is precipitated by addition of a low-boiling paraffin solvent, or paraffin naphtha, such as normal pentane, and is soluble in carbon disulfide and benzene. In certain methods their concentrations are defined as the amount of asphaltenes precipitated by addition of an n-paraffin solvent to the feedstock, for instance, as prescribed in the Institute of Petroleum Method IP-143. The heavy fraction can contain asphaltenes when it is derived from carbonaceous sources such as petroleum, coal or oil shale. There is a close relationship between asphaltenes, resins and high molecular weight polycyclic hydrocarbons. Asphaltenes are hypothesized to be formed by the oxidation of natural resins. The hydrogenation of asphaltic compounds containing resins and asphaltenes produces heavy hydrocarbon oils, that is, resins and asphaltenes are hydrogenated into polycyclic aromatic or hydroaromatic hydrocarbons. They differ from polycyclic aromatic hydrocarbons by the presence of oxygen and sulfur in varied amounts.
Upon heating above about 300-400° C., asphaltenes generally do not melt but rather decompose, forming carbon and volatile products. They react with sulfuric acid to form sulfonic acids, as might be expected on the basis of the polyaromatic structure of these components. Flocs and aggregates of asphaltenes will result from the addition of non-polar solvents, for instance, paraffinic solvents, to crude oil and other heavy hydrocarbon oil feedstocks.
Conventional methods are limited in their efficacy for processing of whole crude oil feeds. For example, fixed-bed reactors require shut-down for catalyst unloading and replacement. This reduces the on stream factor and as a result increases the processing costs of the hydroprocessing units.
It is therefore desirable to provide an improved system and a process for efficient treatment of a whole crude oil to upgrade its quality.